This invention relates generally to dosimeters for measuring radon and more particularly, to a passive radon measuring device having a two-piece housing or diffusion chamber and a mounting insert for positioning both a membrane or filter and a radon track etch or energy sensitive detector within the diffusion chamber
Radon is a colorless, tasteless, and odorless radioactive gas that results from the natural breakdown or radioactive decay of radium. Radon typically is found in high concentrations in soils and rocks containing uranium. It is believed that human exposure to elevated levels of radon can lead to an increased risk of developing lung cancer, depending upon the concentration of radon and the length of exposure.
Recent studies have indicated that radon can accumulate in dangerous concentrations in residences and other structures, and particularly in the lower levels of buildings which typically have poor ventilation and into which radon enters from the surrounding soil. Radon can enter a structure in numerous ways including through the water supply, dirt floors, cracks in masonry floors and walls, floor drains, sumps and similar openings in the foundation of the structure.
Various methods and devices have been employed for detecting radon. The two least expensive and most readily available devices are the charcoal canister and the alpha track detector. Both of these devices are passive devices that are exposed to the air in a home or other structure for a prescribed period of time and are then sent to a laboratory for analysis. Although charcoal canisters are usable for a test period of one to seven days, they are somewhat less reliable than alpha track detectors.
The most basic alpha track detector is constructed as a housing or diffusion chamber in the form of a small cup having a strip of alpha track registration material affixed to the inside of the housing. When the track registration material is exposed to radon or its progeny (radioactive decay products of radon) the alpha particles produced by the radioactive decay of the radon or progency cause minute damage tracks to occur on the material. Such tracks can subsequently be enlarged and made visible by chemical or electrochemical etching, for instance, and the concentration of radon present in a particular test area can be measured.
Although an isolated piece of track registration material can be placed in an environment for detection of radon, it is preferable that some type of housing or diffusion chamber be used, such as the cup housing referred to above. A housing and membrane or filter isolates the registration material from the ambient air, which may contain concentrations of radon progeny, and enables the material to be exposed to radon entering the housing from the environment and the daughters produced in the housing. Additionally, it is desirable to provide a membrane or filter over the entrance to the housing to prevent contamination from other sources. The sensitivity of these types of detectors depends upon the size, shape and material used in their construction. More importantly, the position of the detector strip within the device can also affect the performance and reliability of these detectors. Care must also be taken to prevent tampering with the detector before analysis in the laboratory, as well as maintaining the detector in an optimum position for receiving alpha particles from radon and its progeny within its interior during the test period.
One example of a radon detector having a housing and a filter is disclosed in U.S. Pat. No. 4,518,860. That patent discloses a track registration detector for radon and radon progeny products having a housing with a removable circular apertured closure cap for retaining a strip of track registration material within its interior. The strip is retained within the housing by integrally formed upstanding ribs which form both a pedestal support and a transverse support for the strip and position the strip in juxtaposition with the apertures of the cap with a circular filter sandwiched therebetween. The presence of radon is measured on the side of the strip opposite the filter and apertured cap. The cap includes a solid circular portion in its center to provide a radiation absorber shield for the top surface of the strip. The entrance area of this detector is capable of being blocked by an item positioned adjacent to the apertures, and, since the detector is typically placed with its apertured cap facing upward, can become clogged by dust and dirt particles falling naturally within the ambient air.
Other detectors are of a size most suitable for use as a personal dosimeter or for exploration purposes.
The present invention provides a track etch radon detector which is of relatively uncomplicated construction and yet achieves the desired advantages of permitting free flow of radon within its interior without exposing the track registration material to ambient air, reduces the chance of becoming blocked with foreign material or adjacent items, and which provides a seal of the component parts to resist and/or prevent tampering other than by an authorized testing facility. The construction of the invention has a relatively large sensitive volume and detector area and therefore can be used for the same time period as charcoal canisters but without the inadequacies of such charcoal canisters.